UVa 10763 - Foreign Exchange

#include<iostream>
#include<map>
#include<vector>
using namespace std;

struct Ex_student {
int x, y;
};

struct Ex_cmp {
bool operator () (const Ex_student &k1, const Ex_student &k2) const {
if(k1.x != k2.x)
return k1.x < k2.x;
if(k1.y != k2.y)
return k1.y < k2.y;
return false;
}
};

int main() {
int n;
while (cin >> n&& n) {
bool exsit = true;
vector<Ex_student> s;
map<Ex_student, int, Ex_cmp> student_list;

for(int i = 0; i < n; i++) {
Ex_student student;
cin >> student.x >> student.y;
if(student_list.count(student))
student_list[student]++;
else {
student_list[student] = 1;
}
s.push_back(student);
}

for(int i = 0; i < n; i++) {
Ex_student student;
student.x = s[i].y;
student.y = s[i].x;
if(student_list.count(student)&&
student_list[student] == student_list[s[i]])
; else {
exsit = false;
break;
}
}
exsit ? cout << "YES" << endl
: cout << "NO" << endl;
}
return 0;
}